A light emitting device includes a p-side heterostructure, an n-side heterostructure, an active region disposed between the p-side heterostructure and the n-side heterostructure. An electron blocking layer (ebl) disposed between the p-side heterostructure and the active region comprises an aluminum containing group-III-nitride alloy. An aluminum composition of the ebl decreases as a function of distance along a [0001] direction from the active region towards the p-side heterostructure over a majority of the thickness of the ebl.
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1. A light emitting device, comprising:
a p-side heterostructure;
an n-side heterostructure;
an active region disposed between the p-side heterostructure and the n-side heterostructure and configured to emit light;
an electron blocking layer (ebl) disposed on a growth surface and arranged between the p-side heterostructure and the active region, the ebl having a thickness and comprising an aluminum containing group-III-nitride alloy; and
a p-side superlattice disposed on the electron blocking layer,
wherein an aluminum composition of the ebl changes as a function of distance along a growth direction-from the active region towards the p-side heterostructure over a majority of the thickness of the ebl, and wherein the change in the aluminum composition is dependent on growth surface orientation, wherein the aluminum composition in the ebl decreases along the growth direction for a growth surface orientation having crystallographic indices (hk(
17. A light emitting device, comprising:
a p-side heterostructure;
an n-side heterostructure;
an active region disposed between the p-side heterostructure and the n-side heterostructure and configured to emit light, the active region comprising a III-nitride material; and
an electron blocking layer (ebl) disposed on a growth surface and arranged between the p-side heterostructure and the active region, the ebl having a thickness and comprising an aluminum containing group III-nitride alloy, wherein a change in an aluminum composition of the ebl is dependent on orientation of a growth surface, and
wherein the growth surface orientation is a semi-polar growth surface orientation identified by crystallographic indices (hk(
the growth surface orientation is a semi-polar growth surface orientation identified by crystallographic indices (hk(
21. A light emitting device, comprising:
a p-side heterostructure;
an n-side heterostructure;
an active region disposed between the p-side heterostructure and the n-side heterostructure and configured to emit light, the active region comprising a number of quantum wells separated by barriers, the barriers including a last barrier comprising alzGa1-zN, only a portion of the last barrier being doped with a p-type dopant; and
an electron blocking layer (ebl) disposed on a growth surface and arranged between the p-side heterostructure and the last barrier, the ebl comprising alyGa1-yN, wherein y changes as a function of distance from the last barrier and the change in y is dependent on orientation of the growth surface, wherein y decreases as a function of distance along a growth direction over a majority of a thickness of the ebl for a growth surface orientation identified by crystallographic indices (hk(
wherein a value of z proximate the ebl is greater than the value of z proximate the quantum well.
9. A light emitting device, comprising:
a p-side heterostructure;
an n-side heterostructure;
an active region disposed between the p-side heterostructure and the n-side heterostructure and configured to emit light; and
an electron blocking layer (ebl) disposed between the p-side heterostructure and the active region and having a thickness, the ebl comprising alyGa1-yN, wherein y is graded in a number of discrete steps, a width of the steps changing as a function of distance along a growth direction from the active region towards the p-side heterostructure over a majority of the thickness of the ebl, wherein an aluminum composition of the ebl changes as a function of distance along the growth direction-from the active region towards the p-side heterostructure over a majority of the thickness of the ebl, wherein the change in the aluminum composition is dependent on growth surface orientation, wherein the aluminum composition in the ebl decreases along the growth direction for a growth surface orientation having crystallographic indices (hk(
28. A method of making a light emitting device, comprising:
growing an n-side heterostructure on a polar or semipolar face of a substrate;
growing an active region comprising one or more quantum wells disposed between barriers, the active region configured to emit light during operation of the light emitting device;
growing a p-side heterostructure; and
growing an electron blocking layer on the active region, wherein the electron blocking layer (ebl) is grown on a growth surface and is disposed between the p-side heterostructure and the active region, the ebl having a thickness and comprising an aluminum containing group III-nitride alloy, wherein:
growing the ebl comprises grading an aluminum composition of the ebl dependent on orientation of the growth surface such that the growth surface orientation has crystallographic indices (hk(
4. The device of
5. The device of
6. The device of
7. The device of
8. The device of
10. The device of
11. The device of
12. The device of
14. The device of
15. The device of
16. The device of
an electron quasi Fermi level in the ebl is more than about 0.25 eV from the conduction band energy over a majority of the thickness of the ebl; and
a hole quasi Fermi level in the ebl is less than about 0.1 eV from the valence band energy over a majority of the thickness of the ebl.
18. The device of
19. The device of
20. The device of
22. The device of
23. The device of
an electron quasi Fermi level in the ebl is more than about 0.25 eV from the conduction band energy over a majority of the thickness of the ebl; and
a hole quasi Fermi level in the ebl is less than about 0.1 eV from the valence band energy over a majority of the thickness of the ebl.
24. The device of
25. The device of
26. The device of
27. The device of
29. The method of
growing the active region comprises growing a last barrier between a last quantum well and the ebl and a first aluminum composition in the last barrier proximate the ebl is greater than a second aluminum composition in the last barrier proximate the last quantum well; and
only a portion of the last barrier is doped with a p-type dopant, the portion being farthest from the last quantum well.
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This invention was made with U.S. Government support through the Defense Advanced Research Projects Agency (DARPA) under Army Research Laboratory (ARL) Cooperative Agreement # W911NF-10-02-0102. The Government has certain rights in this invention.
Various embodiments described herein involve a light emitting device, that includes a p-side heterostructure, an n-side heterostructure, and active region disposed between the p-side heterostructure and the n-side heterostructure and configured to emit light. The active region may comprise a III-nitride material with an electron blocking layer (EBL) disposed between the p-side heterostructure and the active region. The EBL comprises an aluminum containing group-III-nitride alloy, wherein an aluminum composition of the EBL decreases as a function of distance along a [0001] direction from the active region towards the p-side heterostructure over a majority of the thickness of the EBL.
Some embodiments involve a light emitting device that includes a p-side heterostructure, an n-side heterostructure, and an active region disposed between the p-side heterostructure and the n-side heterostructure, where that active region is configured to emit light. The light emitting device includes an electron blocking layer (EBL) disposed between the p-side heterostructure and the active region. The EBL comprises AlyGa1-yN, wherein y decreases as a function of distance along a [0001] direction from the active region towards the p-side heterostructure over a majority of the thickness of the EBL.
Various embodiments described herein involve a light emitting device that includes a p-side heterostructure, an n-side heterostructure, an active region disposed between the p-side heterostructure and the n-side heterostructure and configured to emit light, the active region comprising a III-nitride material and an electron blocking layer (EBL) disposed between the p-side heterostructure and the active region. The EBL includes an aluminum containing group III-nitride alloy. In some implementations, the EBL is grown on a surface identified by crystallographic indices (hk(
Some embodiments involve a light emitting device that includes a p-side heterostructure, an n-side heterostructure and an active region disposed between the p-side heterostructure and the n-side heterostructure and configured to emit light, the active region comprising a number of quantum wells separated by barriers, the barriers including a last barrier comprising AlzGa1-zN. An electron blocking layer (EBL) comprising AlyGa1-yN is disposed between the p-side heterostructure and the active region.
In some implementations the EBL is grown on a surface identified by crystallographic indices (hk(
In some implementations, the EBL is grown on a surface identified by crystallographic indices (hk(
Some embodiments are directed to a method of making a light emitting device. According to the method, an n-side heterostructure is grown on a polar or semi-polar face of a substrate. An active region that includes one or more quantum wells disposed between barriers is grown on the n-side heterostructure. The active region is configured to emit light during operation of the light emitting device. A p-side heterostructure is grown with an electron blocking layer (EBL) comprising an aluminum containing group III-nitride alloy grown between the active region and the p-side heterostructure.
In some configurations, the EBL is grown on a surface identified by crystallographic indices (hk(
In some configurations, the EBL is grown on a surface identified by crystallographic indices (hk(
Light emitting diodes and laser diodes can be produced based on larger bandgap semiconductor materials, such as III-nitride materials including AlN, AlGaN, InGaN, InAlGaN, and others. However, doping these materials to achieve high hole concentration is difficult because the activation energy of dopants in these larger bandgap materials is relatively high. Approaches described in this disclosure relate to polarization enhanced doping to increase the hole concentration in one or more layers of light emitting devices formed using polarized crystals, such as the III-nitride family. Although discussed in terms of the III-nitrides, those skilled in the art will understand that the techniques presented herein are applicable to polarization enhanced doping for light emitting devices formed using a variety of polarized semiconductors.
Polarization is an intrinsic property of some crystals, and is related to the distribution of positive charge of the atomic nuclei and the negative charge of the electrons in the crystal. Polarization can be employed to increase or decrease the local concentration of mobile carriers present in semiconductors. This is called polarization enhance doping and can be employed to improve the performance of devices.
Polarization is present in compounds exhibiting the wurtzite structure and comprising atoms having large differences in electronegativity. Examples of such materials include AlN, GaN, InN, and alloys of these materials (AlGaN, InAlN, AlGaInN). Polarization is also present in oxides such as ZnO and BeO. The polarization of an alloy depends on the composition of the alloy. The amount of polarization also depends on the amount of strain present in the material. The polarization that arises from strain is called the piezoelectric polarization, and the polarization that is present even in the absence of strain is called the spontaneous polarization. Strain affects the polarization because it affects the distribution of charge. A very thick freestanding film of AlN will exhibit no strain, and in such a state will exhibit spontaneous polarization only.
A layer of material A will be strained when it is grown on a thick substrate material S if the equilibrium lattice constant of A (in the plane of the interface with S) differs from that of S. In the absence of strain relieving defects the in-plane lattice constant of A is forced to be the same as that of S. In this case we say that A is pseudomorphic to S. For example, if a thin layer of AlGaN is grown on a thick substrate of AlN then the AlGaN will exhibit compressive strain, and the AlGaN layer will exhibit both spontaneous and piezoelectric polarization. The AlN substrate is not strained and will exhibit only the spontaneous polarization of bulk AlN. If a thin layer of AlGaN is grown on a thick substrate of GaN then the AlGaN will exhibit tensile strain. Thus the polarization in AlGaN will depend on whether it is grown on AlN or GaN.
Polarization gives rise to fixed charges at the interface formed between two layers of material with different polarization. Because the polarization charges are fixed, they do not themselves contribute to electrical transport in the crystal, however, the polarization charges can affect the concentration of mobile carriers. The polarization charges are present when the polarization field within the crystal is changing with distance. For example, the discontinuity in the polarization that arises at an interface between two materials having different polarization gives rise to a sheet charge density that is proportional to the change in the polarization across the interface.
When fixed charges are present at the interface, the fixed charges are neutralized by mobile charges in the material. The amount of mobile charge that the polarization charges induce at the interface of two nitride layers is dependent on the composition and structure of the crystal. In the case of AlGaN, for example, the aluminum composition of the AlGaN at the junction determines the amount of polarization and thus the amount of mobile charges induced at the interface. Note that polarization enhanced doping may be applied to carriers of either type. In nitride based light emitting devices, polarization enhanced doping is particularly relevant to enhancing hole concentration due to the difficulty of doping the nitride layers to achieve high p-type donor concentrations and simultaneously providing high conductivity layers.
The active region may include one or multiple quantum wells that are separated by barriers. The last barrier 105a of the active region proximate to the EBL is depicted in
In some embodiments, hole concentration in the electron blocking layer (EBL) 125 can be increased by varying the Al concentration of the EBL. Polarization created by a decreasing Al composition in the EBL as a function of distance can increase the hole concentration in the EBL. At least some of these additional holes are injected into the active region and increase the injection efficiency of the device. EBLs that have a varying Al composition are referred to herein as “graded EBLs.” Note that the Al composition in the EBL may in general have any profile and need not be a linear or ramped profile so long as the Al composition is varying. The Al composition profile may be linear or non-linear, may be stepped, and/or may be monotonic or non-monotonic.
The active region 105 may include quantum wells interspersed with barrier layers, the quantum wells and/or barriers may be formed using any nitride based material system, such as InGaN, AlGaN, InAlN, InAlGaN, or other nitrides. The EBL may be a III-nitride material including InGaN, AlGaN, InAlN, InAlGaN or other aluminum containing group III nitrides. In some cases, the EBL can predominantly comprise AlGaN along with a small amount of In. Indium may be used as a surfactant to facilitate growth of the device and a small amount of In may incorporate in the predominantly AlGaN EBL during fabrication.
The device may alternatively comprise other material systems, e.g., ZnO, or other material systems having a wurtzite crystalline structure and exhibiting sufficiently large polarization fields to cause a significant increase in carrier concentration. The EBL can be doped with p-type donor atoms. For EBLs comprising III-nitride materials, a preferred dopant is Mg and a suitable doping level is a concentration of Mg atoms of about 1019/cm3 to 5×1020/cm3. In some cases, the graded EBL 125 may comprise AlyGa1-yN wherein y decreases from a first, higher Al composition value proximate to the active region 105 towards a second, lower Al composition value proximate to the p-side heterostructure 120 along the growth direction, e.g., the [0001] direction, of the device.
Referring to
The vector normal to the surface, Vn, is the growth direction. In the case where θ=0, a polar orientation, the growth direction is the [0001] direction. In general, devices grown on surfaces identified by crystallographic (Miller) indices (hk(
Devices grown surfaces identified by crystallographic (Miller) indices (hk(
The family of plots shown in
In some cases, the graded EBL may comprise AlyGa1-yN having an Al composition profile corresponding to y proximate the active region in a range of about 0.8 to about 1 and y proximate the p-side superlattice in a range of 0.7 to 0.5. In some cases, the graded EBL comprises In1-yAlyN having an Al composition profile corresponding to y proximate the active region of about 0.93 and y proximate the p-side heterostructure of about 0.81. The reason for the smaller change in the Al composition for InAlN when compared to AlGaN is that the band gap of InAlN changes more quickly with Al composition than does AlGaN. In general, the graded EBL may have an Al composition proximate the active region corresponding to y in a range of about 0.8 to about 1 and an Al composition proximate the p-side heterostructure corresponding to y in a range of about 0.85 to about 0.5. The value of y may decrease by about 0.35 over a majority of the EBL. The final value of y proximate the p-side heterostructure may be in the range of the Al composition at the superlattice composition. The rate of change of the Al composition in the graded EBL may be greater than or equal to about 30% in 10 nm, for example.
The variation in the Al composition in the graded EBL affects the energy band structure within the EBL.
The decrease in the Al composition along the [0001] direction over a majority of the thickness of the EBL provides increased hole concentration in the EBL, increased hole injection into the active region, and reduced flow of electrons past the active region into the p-side heterostructure. The step-wise decrease in the Al composition occurs over a majority of the thickness of the EBL and causes a corresponding step-wise decrease in the conduction band energy in the EBL. The conduction band energy decreases from a first conduction band energy value at a first point proximate to the active region to a second conduction band energy value at a second point proximate to the p-side heterostructure. Note that in
In this case, difference between the first conduction band energy value (1st CB energy≈0.25 eV) and the second conduction band energy value (2nd CB energy≈−0.3) is about 0.55 eV. In general, using a graded EBL as discussed herein, a decrease between the first conduction band energy value and the second conduction band energy value may have a range of about 0.5 eV to about 1.0 eV where the decrease occurs over a majority, e.g., more than 50% or even more than 86% of the thickness of the EBL. In some cases the first conduction band energy value may be 0.4 eV greater than the conduction band energy value in the quantum wells. The higher value of the first conduction band energy in comparison to the conduction band energies the active region is necessary to inhibit flow of electron current past the active region into the p-side heterostructure.
The valence band energy increases from a first valence band energy value (1st VB energy≈−5.87 eV) proximate to the active region to a second valence band energy value (2nd VB energy≈−5.4 eV) and then remains substantially constant until reaching the p-side heterostructure. Thus, the increase in the valence band energy occurs over a minority of the EBL thickness, e.g., less than 25% of the thickness of the EBL. In the illustrated example, the difference between the first valence band energy and the second valence band energy is about 0.37 eV. In general, using a graded EBL as discussed herein, a difference between the first valence band energy value and the second valence band energy value may have a range of about 0.2 eV to about 0.5 eV, wherein the increase occurs over less than 25% of the EBL thickness.
The higher potential of the conduction band at portion 510a of the conduction band proximate the active region serves to block the escape of at least some electrons injected from the n-side of the device. Electrons that remain longer in the active region have greater likelihood of combining with holes to create light. Additionally, the step-wise ramped Al composition profile of the EBL provides polarization enhanced hole concentrations in the EBL. As a result of grading the Al composition, the valence band potential 520 seen by the holes is predominantly flat at the portion 520b across a majority (greater than 50%) of the valence band proximate the p-side superlattice, and includes only a narrow variation in potential in the portion 520a across a minority (less than 25%) of the valence band proximate the active region. Holes in the valence band can achieve energy to overcome this narrow barrier or tunnel through the barrier to reach the active region. As is apparent from
As can be appreciated from the plots shown in
It will be appreciated from comparison of
The direction of the Al composition grading is also a factor.
Depending on the direction of the grading of the Al composition, a different polarization charge is created at each of the interfaces in the graded section as the composition of Al is changed. Decreasing the Al composition along the [0001] direction from the active region towards the p-side, as in the graded EBL examples discussed herein, results in a sign of charge that is different from the sign of charge that results from increasing Al composition along the [0001] from the active region towards the p-side. The graded EBL comprises a decreasing the Al composition in the [0001] direction from the active region towards the p-side which produces a high hole concentration and a relatively low electron concentration. In comparison, an oppositely graded EBL, as described in connection with
At higher current densities, in the device having a non-graded EBL (or oppositely graded EBL), a large number of electrons are being injected into the device. However, the potential barrier presented to the injected electrons by the non-graded or oppositely graded EBL is insufficient, thus many of the injected electrons overshoot the active region and are not available for recombination in the active region to produce light. In contrast, for graded EBLs at the higher current densities, the electrons are blocked by the higher potential conduction band barrier of the graded EBL, and thus are retained in the active region to recombine with holes, enhancing the efficiency of the device.
In some cases, a graded EBL may be used in conjunction with a last barrier of the active region that has a spatially varying Al composition. The Al composition in the last barrier may change in a series of steps or change continuously. The last barrier of the active region is the barrier of the active region that is closest to the EBL. Referring back to
A graded last barrier may comprise AlzGa1-zN, where z increases in the [0001] direction from first value of z proximate the last quantum well to a second value of z proximate to the EBL. The graded last barrier can be used in conjunction with a graded EBL to reduce the loss of electrons and holes due to recombination in the last barrier. For example, in polar configurations, the Al composition in the graded last barrier can increase step-wise in the [0001] direction from the last quantum well in the active region towards the EBL, where the z increases from a first value proximate the last quantum well to a second value proximate the graded EBL across at least a portion of the thickness of the last barrier. As previously mentioned, semi polar configurations may be implemented. When a graded EBL is used in conjunction with the graded last barrier, the graded EBL decreases from a first composition of aluminum to a second composition of aluminum along the growth direction of the device from the last barrier towards the p-side heterostructure. The graded last barrier may exhibit any Al composition profile, so long as the Al composition is generally increasing from the last quantum well towards the EBL along the growth direction. The thickness of the last barrier may be in a range from about 4 nm to about 20 nm. In one configuration, the graded last barrier has a thickness of about 7 nm and an Al composition that increases in three steps—a first step 2 nm in width having z=0.54, a second step 3 nm in width having z=0.7 and a third step 3 nm in width having z=0.8.
In some embodiments, a portion of the last barrier is doped, e.g., with a p-type dopant such as Mg. It is desirable to keep dopants out of the quantum wells, and if dopants are incorporated in the last barrier during growth then these dopants might diffuse into the quantum wells. To prevent dopants from diffusing to the quantum wells, in some embodiments, only some (but not all) or none of the barriers are doped with a p-type dopant, including the last barrier. However, in some cases it can be helpful to dope at least a portion of the last barrier with a p-type dopant to improve hole injection into the quantum wells. The doped portion of the last barrier, if present, would be that portion furthest from the quantum wells.
Systems, devices or methods disclosed herein may include one or more of the features, structures, methods, or combinations thereof described herein. For example, a device or method may be implemented to include one or more of the features and/or processes described herein. It is intended that such device or method need not include all of the features and/or processes described herein, but may be implemented to include selected features and/or processes that provide useful structures and/or functionality.
In the following detailed description, numeric values and ranges are provided for various aspects of the implementations described. These values and ranges are to be treated as examples only, and are not intended to limit the scope of the claims. For example, embodiments described in this disclosure can be practiced throughout the disclosed numerical ranges. In addition, a number of materials are identified as suitable for various implementations. These materials are to be treated as exemplary, and are not intended to limit the scope of the claims.
The foregoing description of various embodiments has been presented for the purposes of illustration and description and not limitation. The embodiments disclosed are not intended to be exhaustive or to limit the possible implementations to the embodiments disclosed. Many modifications and variations are possible in light of the above teaching.
Choi, Suk, Chua, Christopher L., Johnson, Noble M., Yang, Zhihong, Northrup, John E., Cheng, Bowen, Wunderer, Thomas
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